1. Field of the Invention
The present invention relates to composite laminates and to methods for manufacturing the same. In particular, the present invention relates to a composite laminate which shrinks less during firing and to a method for manufacturing the same.
2. Description of the Related Art
In recent years, there have been advances in the reduction in size and weight of chip components. Reduction in size and weight is also required for circuit boards for mounting the chip components. Glass ceramic multilayered circuit boards are useful to meet this need because the glass ceramic multilayered circuit boards allow high-density wiring and reduction in thickness, resulting in reduction in size and weight.
Glass ceramic multilayered circuit boards are generally formed by a sintering process, and they shrink during sintering in a direction perpendicular to the main faces of the boards (longitudinal shrinkage) and in a direction parallel to the main faces (transverse shrinkage). Thus, the current glass ceramic multilayered circuit boards have dimensional variations of approximately ±0.5%. Glass ceramic multilayered circuit boards having cavities for mounting necessary electronic components exhibit noticeable variations.
Japanese Unexamined Patent Application Publication Nos. 5-102666 and 7-330445 disclose methods for making glass ceramic multilayered circuit boards having high dimensional accuracy. Also, Japanese Unexamined Patent Application Publication No. 6-329476 discloses a method for making a glass ceramic multilayered circuit board having cavities. In each method, green sheets which cannot be sintered at the sintering temperature of a glass ceramic compact are laminated on one side or two sides of the glass ceramic compact, and powdered layers of the green sheets are removed after firing.
In such a process, an additional process is required to remove the powdered layers. Moreover, it is difficult to simultaneously fire conductive films, which are preliminarily formed on an unsintered glass ceramic compact, in the firing process. The resulting glass ceramic multilayered circuit board, after removal of the powdered layers, may have a large degree of surface roughness.
In a method disclosed in Japanese Unexamined Patent Application Publication No. 9-266363, a laminate of glass ceramic layers and alumina layers is fired to sinter only the glass ceramic layers so that the glass component contained in the glass ceramic layers penetrates unsintered alumina layers to bind the alumina layers. The glass component in the glass ceramic layers, however, does not penetrate the entity of the alumina layers in this method. Therefore, unbounded portions of the alumina layers are removed and the surfaces are polished before conductive films for circuit patterns are formed.
Although the surface roughness is reduced by the removing and polishing steps in this method, a removing step is required and conductive films cannot be formed on surfaces of the circuit board by simultaneous firing together with the glass ceramic layer.
In a method disclosed in Japanese Unexamined Patent Application Publication No. 5-136572, green sheets which are not sintered at a sintering temperature of a glass ceramic compact, are stacked on one side or two sides of the glass ceramic compact so as to sinter only the glass ceramic compact. Resin is loaded into powdered layers of the unsintered green sheets. This method does not require a step for removing the unsintered powdered layers, but does require a step for loading the powder into the unsintered powdered layers.